The present invention relates to digital transmission via satellite to receivers wich are designed for receiving a signal according to the ETS 300 421 standard (a digital television broadcasting standard via satellite in the Ku bands, called DVB-S and developed by the DVB, Digital Video Broadcasting group).
This invention is particularly useful when it is necessary to dispatch digital flows of small flow rates (about 6 Mbits/sec, or less) from a plurality of distant sources belonging to independent entities, towards a multiplexing point in order to form therein the final broadcasting multiplex according to the ISO/IEC 13818-1 standard (system part of the MPEG2 standard developed by the MPEG, Moving Picture Expert Group), while avoiding an architecture involving a terrestrial and centralized transit point.
This invention relates to an improvement of the construction described in French Patent Application filed under No 96 11431 on Sep. 19, 1996 in the name of the Applicants.
The main improvement to the above mentioned construction which is the subject matter of the present invention, relates to introducing a Time Division Multiple Access (TDMA) working mode. Its usefulness is hereinafter justified.
The compressed digital television services according to the MPEG2 standard necessitate flow rates of 5 Mbits/sec. for a so-called xe2x80x9cbroadcastxe2x80x9d video quality, or about a total of 6 Mbits/sec., if a plurality of high quality audio paths and auxiliary data are added. It is sufficient to use a basic architecture as described in the above mentioned patent application and in which six stations are in the course of continuously broadcasting elementary flows of about 6 Mbits/sec. towards a satellite-borne multiplexer.
However, various multimedia services, under current development and called xe2x80x9cemergentxe2x80x9d, may be satisfied with very smaller flow rates, i.e. 3 Mbits/sec. for distance teaching applications or 2 Mbits/sec. for medical imaging applications, and even some hundreds of kilobits/sec. for a vision-conference according to ISO H.261 and H.264 standards. These services can be transmitted under the shape of freely structured data flows, or yet xe2x80x9cprivate dataxe2x80x9d (according to terminology of the ISO/IEC 13818-1 standard) in a MPEG2 multiplex. These flows are designed to be processed by a computer and may be received by digital television receivers having a so-called data interface on which a data process equipment can be connected. These flows may also be received in a personnal computer fitted with an extension card that is plugged on the extension bus of the computer.
In a satellite-borne system according to FIGS. 1 and 2 of the above mentioned patent application, in which each broadcasting station is assigned with a different frequency channel and continuously transmits in this channel, the great plurality of flow rates to be offered for multimedia services and the great number of demodulators that are necessary make difficult its construction at the level of the satellite-borne module and, in any case, make not very effective a working of the satellite-borne system because the uncertainty of the preponderance of a range of given flow rates.
The TDMA working mode, given the flow rate flexibility therof, responds in an efficacious and appropriate manner to the needs of multimedia services while maintaining the complexity of the satellite-borne equipment into reasonable proportions. In this working mode, earth stations, while referring to a common time base, will transmit on a same frequency, in a sequential order and without time overlaping, digital bursts at a higher flow rate than the flow rate of their respective sources.
In a transmission through a so-called transparent satellite repeater, the time base of a TDMA system is provided by one or more so-called reference stations which transmit, towards the satellite, leading and trailing frame marking signals. The other stations, by a computing operation or a continual approach technic, deduce their transmitting instants from the receiving instants of the frame marking signals coming from the reference stations.
This principle cannot be applied to a system with demodulation and multiplexing in packets on board a satellite because of the necessary presence of buffer-memories used for realigning the packets coming from a plurality of transmitters. Differently speaking, the instant of arrival on earth of a packet, on the going down path, is not enough precisely representative of the instant of arrival of this packet onto the satellite on the going up path. An other mechanism enabling the earth stations to know the instants of arrival of their packets at the level of satellite reception must be definied.
The present invention concerns the construction of a TDMA system, in a system with demodulation and multiplexing by packets on board, construction in which:
1. One or more of the links on the going up paths 1a, 1b, . . . 1n of a system according to FIGS. 1 and 2 of the above mentioned patent application, may be configured in TDMA transmission mode, while the other links may remain in a continuous transmission mode (hydrid system).
2. The TDMA or continuous configuration of a link is made without the working of the other links be affected in some manner (principle of separation of the links).
3. The structure in MPEG2 packets of the signal that is transmitted via the satellite is used for the definition of the common time base and TDMA frames (TDMA system without reference stations).
4. Special packets are used for enabling the earth stations to make the acquisition and maintenance of the synchronisation with the time base transmitted by the satellite-borne module according to a particular protocole between them and the receiving part of the satellite-borne module (synchronization by means of a satellite-borne software).
5. Activation or termination of the transmission of a TDMA station is made without coordination with the other stations sharing a same link (principle of autonomy).
6. Adjonction of the TDMA mode is modular and necessitates only a few changes in the equipment of the earth broadcasting stations with respect to a continuous transmission mode (principle of modularity).
7. Adjonction of the TDMA mode has no occurrence on the compatibility with the earth receivers designed for receiving the DVB-S signal.
8. The basic parameters, frame length, guard-time and synchronisationprotocole, enable a simple embodiment of the satellite-borne module, without significative loss in effectiveness (principle of minimal complexity).
According to the invention, the device for broadcasting digital information via satellite from a plurality of earth stations, comprises:
at least one set of links sending packet formated digital information to the satellite, on at least two different frequencies;
at least, one satellite-borne module which receives, multiplexes and re-formates the information on only one downlink;
at least one of the links which works in a Time Division Multiple Access (TDMA) mode.
According to other features of the invention:
the signal that is transmitted via satellite is a time multiplex signal containing all the useful information, individually transmitted toward via satellite by a plurality of earth stations and rigorously compatible with receivers designed for a reception of signals according to a standard of digital television broadcasting via satellite in the Ku bands, called DVB-S,
one or more of the links of the going up path of a transmission system of an earth station is (are) configured in TDMA transmission mode while the other links remain in continous transmission mode,
the TDMA or continuous configuration of an uplink is made without a working of the other links be affected thereby, by providing an increase of the transmission flow rate on the going up path of a link with respect to the continuous mode working on this link, and this increase is used for accomodating the preamble and guard-time functions of the transmission in a burst mode;
the structure of MPEG2 packets of the signal transmitted via the satellite is used as follows for a definition of the TDMA frames, these TDMA frames being generated on board the satellite:
in lack of received data, the multiplexer generates jamming packets, the program identification (PID) of which takes a single value corresponding to each uplink;
when an uplink is configured in TDMA mode, the multiplexer makes a numbering, in a cyclic manner, of the order of the generated packets, a 5-bit field for numbering the jamming packets, in the useful part of the MPEG2 packet, is used for this purpose;
when a burst in a going up station, among n
stations, comes to the satellite in phase with a portion of TDMA frame, the DVB packets of this burst will be substituted to the jamming packets for this portion of TDMA frame.
the activation or termination of the transmission of a TDMA station is made in an autonomous manner without coordination with the other stations sharing a same link;
a short burst and predetermined packets are used for enabling the earth stations to make a fast, reliable and precise acquisition according to a protocole between them and a receiving part of the satellite-borne module;
basic parameters, frame length, guard-time and synchronisation protocole and the like, are chosen so to enable a simple implementation of the satellite-borne module;
a frame structure transmitted via the satellite comprises a fixed number, equal for example to 48 DVB packets;
a 52/48 ratio is used for the flow rate increase on the uplinks that are re-configured in TDMA mode;
adjonction of the TDMA mode is a modular adjonction, whereby only a few changes are made in the equipment of the earth transmission stations with respect to the continuous transmission mode;
a compatibility of the multiplexed signal on the downlink is made with earth receivers that are designed for receiving signals according to the DVB-S standard with adjonction of the TDMA mode.
Various other features of the invention will moreover be revealed from the following detailled description.